1. Field of the Invention
The present invention relates to a charging device for charging an electric accumulator formed by a plurality of series-connected electric accumulator cells.
2. Description of the Related Art
More and more electric vehicles such as electric cars, fuel-cell-powered cars and hybrid cars have been putting into practical use in order to achieve low emissions, low noise and low gasoline consumption. Such electric vehicles are each equipped with a high voltage battery for driving a running motor. The high voltage of the high voltage battery is obtained by connecting a plurality of low voltage battery cells in series. Due to the high voltage of the high voltage battery, the current flowing through the running motor can be reduced compared with that of a low voltage battery when driving the running motor at the same power level, and therefore the weight of electric wires can be reduced.
Because the characteristic of each of the plurality of series-connected battery cells will vary while repeating charged and discharged, difference in the charging amount will be gradually caused from cell to cell. At the time when the charging amount of any one of the plurality of battery cells reaches a charging upper limit, charging operation has to be stopped even if the other battery cells have not been fully charged; at the time when the charging amount of any one of the plurality of battery cells reaches a charging lower limit, the discharging operation has to be stopped. In other words, since the series-connected battery cells early reach the charging upper limit or charging lower limit, usable charging capacity of the series-connected battery cells is actually decreased. Further, as a failure of the battery, a fine short circuit may occur. In the case where the fine short circuit occurs, the failed battery cell will discharge more quickly than the other battery cells.
To equalize the charging amount for each of the battery cells, there is a proposal in which a series circuit formed by a resistor and a semiconductor switch is connected to both ends of each battery cell to allow each battery cell to appropriately discharge (see Japanese Patent Laid-Open Publication No. 2000-92732, Japanese Patent Laid-Open Publication No. 2001-37077 and Japanese Patent Laid-Open Publication No. 2003-70179). Further, there is another proposal in which a transformer winding and a switching element are connected to each battery cell in order to equalize the charging voltage for each of the battery cells (see Japanese Patent Laid-Open Publication No. 2002-223528 and Japanese Patent Laid-Open Publication No. 2001-339865). In the technique that uses the resistor to discharge each of the battery cells, it is necessary to employ a high breakdown voltage semiconductor switch and a complicated supplementary circuit. Further, in the technique that uses the transformer to achieve insulation, it will be difficult to achieve integration and miniaturization since the transformer has relatively large volume.
In order to reduce the aforesaid problems, there is another proposal in which a capacitor is connected between each of the battery cells and an AC power source to insulate the battery cell from the AC power source, and the variable voltage of the AC power source is superposed on the voltage between the both ends of the capacitor to charge the battery cell (see Japanese Patent Laid-Open Publication No. Hei 11-32443).
However, in the art disclosed in Japanese Patent Laid-Open Publication No. Hei 11-32443, since the both electrodes of each of the series-connected battery cells are insulated from the both electrodes of the AC power source, in the case where a common mode noise is applied between the battery cell and the AC power source (which is a periodical power source), there is concern that a noise voltage will be brought into the battery cell when performing charge. Further, in the art disclosed in Japanese Patent Laid-Open Publication No. Hei 11-32443, a single AC power source is used to simultaneously supply the same voltage to the plurality of battery cells (the electric accumulator cells).